Process for the production of wrought nickel strip and sheet of low hardness



United States Patent 3 268,368 PROCESS FOR THE PRODUCTION OF WROUGHT NICKEL STRIP AND SHEET OF LOW HARDNESS Vladimir Nicolaus Mackiw and Naoyuki Yoshida, Fort Saskatchewan, Alberta, Tibor Krantz, Vancouver, British Columbia, and Robert W. Fraser, Fort Saskatchewan, Alberta, Canada, assignors to Sherritt Gordon Mines Limited, Toronto, Ontario, Canada, a company of Ontario No Drawing. Filed Oct. 21, 1963, Ser. No. 317,780

, 4 Claims. (Cl. 14811.5)

This invention relates to a process for the production of high purity wrought nickel shapes, particularly nickel strip or sheet, of high density, low hardness and high ductility.

Conventional processes for the production of nickel strip or sheet involve a sequence of rolling, heating and annealing operations. When nickel, in the form of a billet :or ingot, is used as the starting material, the preliminary steps involve a sequence of operations to reduce the starting shape to the form of strip or sheet, fol-lowed by hot and/or cold rolling to reduce the shape to a predetermined thickness.

When the starting material is in the form of finely divided nickel particles, the particles are fed into the roll gap of a conventional, r-oll compacting unit to produce a green strip or sheet of a density of from about 50% to 95% of the theoretical density, i.e., about 8.902 grams per cubic centimetre :at 20 C. The green strip or sheet is of sufiicient mechanical strength to be selfsupporting and to withstand a limited amount of handling. The green strip or sheet is sintered in a heated furnace, usually a reducing atmosphere, and then treated by a sequence of hot and/or cold working steps, with or without intermediate annealing stages, to increase the density to approximately 100% of the theoretical density :and to reduce it to a predetermined thickness.

Nickel strip or sheet produced by these conventional processes is strong, relatively ductile and generally has a hardness of the order of from about 35 to 45 measured by the Rockwell 30T hardness scale, regardless of whether it is produced from ingots or bil-lets or from nickel metal particles. Such nickel strip or sheet has many uses, particularly in the electronics industry.

The high hardness of nickel strip or sheet is a disadvantage in certain uses of the material. For example, nickel coinage is produced from blanks stamped from nickel strip or sheet of required thickness. The blanks are then die stamped, obverse and reverse, with the denomination of the particular coin and designs which are characteristic of the country which issues them. It is difficult to obtain sharply defined impressions from the die when the blanks have a hardness such as that produced by conventional rolling processes, that is, from about 35 to 45, measured by the Rockwell 30T hardness scale. Also, the hardness of the blanks causes rapid wear, and even breakage, of the dies.

We have found that strong, ductile nickel strip which has a hardness below 35, Rockwell 30T hardness scale, can be produced from nickel metal particles of random sizes smaller than about 300 microns, provided specific conditions, described in detail hereinafter, are observed in the sintering and hot and/or cold working steps to which a green strip or sheet is subjected in compacting it to its theoretical density and reducing it to its required thickness. For the purpose of the following description, the term low hardness is taken as referring generally to material which has a hardness below about 35 Rockwell 30T.

The process can be simply stated. Nickel metal particles are fed into the roll gap of a conventional roll 3,268,368 Patented August 23, 1966 "ice compacting unit at a predetermined rate to produce a green strip or sheet of desired thickness and adensity within the range of from about 50% to 95% of the theoretical density. Green strip or sheet produced in the roll compacting unit is sintered in a controlled atmosphere at a temperature within the range of from about 1500 F. to about 2100 F. to lower the sulphur and carbon contents of the strip or sheet to safely below specific maximum levels. The sintered strip or sheet is then hot rolled at a temperature about or slightly above the recrystallization temperature of the nickel particles to increase the density to about 100% of its theoretical density. The strip or sheet of substantially 100% density is cold rolled and/ or otherwise cold worked at about ambient temperature to reduce its thickness and/or to produce semi-finished or finished products. The cold worked material is heated to a temperature within the range of from about 1200 F. to about 1800 F. and then cooled to atmospheric temperature in a protective atmosphere which prevents re-oxidation. Wrought nickel produced by this process is of high purity, of substantially 100% density, of which ductility, and of low hardness, within the range of from about 20 to 35, Rockwell 30T hardness scale.

Generally speaking, the process is independent of the source of the nickel powder used as a starting material. That is, the powder can be produced by conventional pyrometallurgical, or hydrometallurgical processes. The powder may contain minor amounts, in the order of 0.2% or less, of metallic impurities generally found in association with nickel, such as cobalt, iron and copper, for example. Other impurities such as carbon and sulphur may also be present in the starting material but must be reduced below certain specific minimum levels as described hereinafter. It is, of course, preferable to select a nickel powder which is of high purity and which readily compacts to form a strong green strip when rolled in a conventional metal powder rolling mill. We have found that nickel powder produced by precipitation from a solution in which it is present as a dissolved salt by reacting the solution with a reducing gas at elevated temperature and pressure is particularly suitable. Nickel powder produced in this manner is of high purity, generally over 99.8% pure nickel, and is readily available in grades which are particularly suitable for roll compacting. The powder particle size should be below about 300 microns and preferably at least about 40% should be in the size range of about 10 microns to about 44 microns.

The finely divided nickel powder is formed into a green strip by compacting it in a conventional metal powder rolling mill. Such a mill comprises, for example, a feeding device arranged to feed the metal powder into the roll gap of a pair of horizontally disposed rolls spaced apart a predetermined distance to compact the particles into a green strip or sheet of desired thickness. We have found that it is important to produce a green strip with a uniform, closely controlled weight per unit area profile across its width. If this characteristic is not properly maintained, the strip is difficult to handle in the subsequent steps, particularly in the hot rolling step. Devices are known for feeding the nickel metal particles to the roll gap of the roll compacting unit and for passing them through the roll gap to produce a strip or sheet having a desired weight of particles per unit of profile area.

The green strip or sheet leaving the rolls has a density of from about to about of the theoretical density of the particulate matter of which it is formed. It is self-supporting and possesses sufficient mechanical strength that it can be passed directly to the sintering step.

In the sintering step, the green strip is passed into afurnace which is maintained at the desired temperature and in which the atmosphere is controlled to obtain efficient removal of excess sulphur and/ or carbon. In this step, a single green strip may be treated or one or more green strips may be superimposed one on top of the other to form a thick, laminated strip. By this latter technique, the thickness of the wrought strip can be controlled independently of the thickness limitations imposed by the metal powder rolling mill.

It is essential to control the atmosphere in the Sintering furnace to lower the sulphur content of the strip to below about 0.003% by weight and the carbon content below about 0.005% by weight.

Sulphur content is reduced to below the minimum critical level by flowing a continuous stream of hydrogen through the sintering furnace. The velocity of the hydrogen gas through the sintering furnace is an important factor in the efficient desulphurization of the metal strip. This velocity should be above about 300 centimetres per minute, the optimum and maximum velocity being determined by operating economics in each particular case. If the hydrogen gas velocity is below the lower level mentioned, the desulphurization proceeds at an uneconomically slow rate, if it proceeds at all.

When the starting material contains carbon in excess of the permissible amount, any excess must be removed. Dry hydrogen is relatively ineffective in removing carbon contamination from the strip; thus, it is necessary to provide, in addition to the hydrogen required for sulphur removal, a reagent that will react with the carbon content of the nickel and effectively remove it. The preferred procedure s to provide water in the hydrogen. The exact moisture content of the hydrogen is not critical, but the time required for carbon removal increases as the dryness of the gas is increased. For most purposes we have found that hydrogen having a dew point of 50 to 80 F. is suitable; however, this may be lower when low-carbon nickel powder is used or higher if high carbon nickel powder is used. An alternative procedure is to provide a small amount of carbon dioxide in the sintering atmosphere. Satisfactory results are obtained when the hydrogen fed into the furnace contains about double the stoichiometric carbon dioxide concentration required for decarbonization.

The sintering furnace temperature is maintained in the range of from about 1500 F. to about 2100 F. The strip or sheet is treated under these conditions for a time sufficient to reduce the sulphur content to below about 0.003% by weight and the carbon content of the metal to below about 0.005% by weight. We have found that in most cases a time period of from about minutes to about 45 minutes is sufficient to reduce the sulphur and carbon contents to the desired levels, the exact time depending on operating conditions and the amount of sulphur and/or carbon contamination in the starting material.

The type of furnace used is not critical except that it must be designed to provide a constant and uniform temperature throughout, and it must allow for the free flow of hydrogen gas around or over the strip at a velocity preferably greater than about 300 centimetres per minute. We have found that a coil furnace is quite suitable where a single strip of compacted metal is being treated. However, if it is desirable to superimpose one or more green strips on top of one another to produce thick strip, we have found that a long horizontal furnace is most suitable. In using such a furnace, the stacked green strips are cut into lengths equal to the effective heat-treating length of the furnace, and each length is sintered for the required period of time on a batch basis. Sintering under the controlled atmosphere is continued until the sulphur and carbon contents of the strip have been reduced to safely below the maximum amount that can be tolerated. When the sintering step is complete, usually within about 15 to 45 minutes, the strip is fed, in a protective atmosphere, directly into a conventional hot rolling mill where it is roll compacted to substantially of the theoretical density. When stacked strips are subjected to this hot rolling operation, they become welded together, forming a thick coherent strip free from any indication of lamination.

The hot rolled strip which, at this stage, is relatively hard having, for example, a hardness in the range of from about 45 to 50 Rockwell 30T hardness scale may be passed to a first annealing step. In this step, the strip is heat-treated at a temperature within the range of from about 800 F. to about 2200 F. under a deoxidizing atmosphere, preferably hydrogen gas. The anneal is con tinued at least until the surface oxidation formed during the Sintering and hot rolling steps is removed. The time required usually is from about 5 to about 60 minutes. It is important that the surface oxidation be removed at this stage, otherwise the oxides will be occluded in the surface of the material during the subsequent cold workmg steps, and the primary function of the first anneal is to de-oxidize the hot worked strip; although we have found that this anneal also has an effect on the final product hardness.

Thus, this first anneal step is not required if surface oxidation of the hot rolled strip is prevented by other means such as by maintaining the strip in a reducing atmosphere while it is being fed into the hot rolling mill, immediately when it emerges from the hot rolling mill, and while it is being cooled to cold working temperature.

The cooled de-oxidized strip is then passed to a cold working step where it is cold rolled to reduce it to the desired thickness and further Worked to produce the rough form of the end product. For example, where the final products are nickel coinage blanks, the strip is cold rolled to the thickness required for the blanks; then the blanks are punched out of the strip using conventional equipment.

The material passed from the cold working steps is hard and has low ductility as a result of accumulated work hardening, and the final annealing step is required to produce a product with the desired hardness and ductility. This final step, which is referred to herein as the final anneal, is carried out under a protective atmosphere and at a temperature within the range of from about 1200 F. to about 1800 F. The exact temperature will depend on the hardness desired for the final product. We have found that hardness of the annealed product can be largely controlled by controlling the temperature of the final anneal. Generally, the hardness of the annealed product decreases with increasing temperature. The time required for the final anneal depends on the size and shape of the wrought products being treated. For most purposes from about 15 to about 60 minutes is sufficient. For optimum results, it is necessary to con duct the final anneal at a temperature within the range of from about 1200 F. to about 1800 F. for the minimum length of time required to produce uniform properties in the particular product being treated.

The products from the final anneal step are semifinished forms of wrought nickel of exceptionally low hardness which can be readily cold worked to produce finished products.

The following example illustrates the operation of the overall process:

Example 1 Nickel metal particles having properties set out in Table l were compacted in a conventional roll compacting unit:

Chemical analysis:

Ni 99.80 Co 0.09 Fe 0.04 Cu 0.05 S 0.022 C 0.004

The density of the green strip produced by the roll compacting unit was 84% of the theoretical density, and the'strip had sufficient green strength to be self-supporting for handling in the sintering operation.

7 The green strip was sintered at 1800 F. for 35 minutes in a stream of hydrogen flowing through the furnace at 7200 centimetres per minute. The sintered strip contained 0.003% sulphur and 0.004% carbon. The surface of the strip was discoloured from surface oxidation.

The sintered strip was passed, in a protective atmosphere, to a hot rolling mill comprised of two horizontally mounted, oppositely positioned rolls. The strip emerging from these rolls had a density substantially 100% of the theoretical density.

The hot rolled strip was heated at 2000 F. for minutes under a hydrogen atmosphere. The annealed strip had a bright surface, indicating that surface oxidation had been removed, and a hardness of 40 (Rockwell 30T).

The de-oxidized strip was cold rolled to a thickness of 0.058 inch and coinage blanks were punched from the strip with conventional punching equipment. It was found that the work hardening resulting from these cold working operations had increased the hardness of the nickel strip to 75 (Rockwell 30T).

The coinage blanks were annealed, under a hydrogen atmosphere, for 30 minutes at 1200 F. The resulting product had a hardness of (Rockwell T).

Example 2 reduction of nickel from an ammoniacal nickel ammonium sulphate solution. The analyses of these powders is shown below.

TABLE II A, uy new, percent percent percent Samples of these powders were processed as follows: (1) The powders were roll compacted to produce green strips;

(2) the green strips were sintered in dry hydrogen or wet hydrogen (dew point 55 F.) for various times at 1850 F. as indicated;

(3) the sintered strips were hot rolled reduction);

(4) the hot rolled strips were annealed at 2200 F. for 30 minutes in hydrogen;

(5) the anneal strips were cold rolled to 0.058 inch (40% reduction);

(6) the cold rolled strips were annealed at 1400 F.

for 30 minutes.

The carbon content and hardness of each sample after treatment is shown in Table III.

These results illustrate how final product hardness increases with increasing carbon content.

It will be understood that modifications may be made in the improved process of this invention without departing from the scope of the invention defined by the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. The process of producing nickel sheet and strip having a density of about of the theoretical density and a hardness below about 35 Rockwell 30T hardness scale which comprises the steps of:

(a) roll compacting nickel particles of random sizes smaller than about 300 microns to produce a green strip or sheet having a density less than 100% of the theoretical density;

(b) sintering said strip in a flowing wet hydrogen atmosphere at a temperature within the range of from' about 1500 F. to about 2100 F. to lower the sulphur content of said strip to below about 0.003% by weight and the carbon content of said strip to below about 0.005% by weight;

(c) hot rolling said sintered strip or sheet to increase the density thereof to about 100% of the theoretical density;

(d) cooling said strip or sheet under a protective atmosphere to about normal room temperature;

(e) cold working said strip or sheet; and

(f) thereafter heating said strip or sheet at a temperature within the range of from about 1200" F. to about 1800 F. for a period of time suflicient to reduce the hardness thereof to a selected value below 35 Rockwell 30T hardness.

2. The process of producing nickel sheet and strip having a density of about 100% of the theoretical density and hardness below about 35 Rockwell SOT hardness scale which comprises the steps of:

(a) roll compacting nickel particles of random sizes smaller than about 300 microns to produce a green strip or sheet having a density less than 100% of the theoretical density;

(b) sintering said strip in a flowing wet hydrogen atmosphere at a temperature within the range of from about 1500 F. to about 2100 F. to lower the sulphur content of said strip to below about 0.003% by weight and the carbon content of said strip to below about 0.005% by weight;

(c) hot rolling said sintered strip or sheet to increase the density thereof to about 100% of the theoretical density;

(d) cooling said strip or sheet to about normal room temperature;

(e) heating said strip in a reducing atmosphere at a temperature within the range of from about 800 F. to about 2200 F. for sufficient time to remove surface oxidation;

(f) cold working said strip or sheet; and

(g) thereafter heating said strip or sheet at a temperature within the range of from about 1200 F. to about 1800 F. for a period of time sufiicient to re 7 duce the hardness thereof to a selected value below 35 Rockwell 30T hardness.

3. The process of producing nickel sheet and strip having a density of about 100% of the theoretical density and hardness below about 35 Rockwell 30T hardness scale which comprises the steps of:

(a) roll compacting nickel particles of random sizes smaller than about 300 microns and obtained by hydrogen reduction from aqueous media to produce a green strip or sheet having a density less than 100% of the theoretical density;

(b) sintering said strip in a reducing atmosphere of wet hydrogen having a dew point not less than about 50 F. at a temperature within the range of from about 1500 F. to about 2100 F. to lower the sulphur content of said strip to below about 0.003% by Weight and the carbon content of said strip to below about 0.005% by weight;

() hot rolling said sintered strip or sheet to increase the density thereof to about 100% of the theoretical density;

(d) cooling said strip or sheet under a reducing atmosphere to about normal room temperature;

(e) cold working said strip or sheet to produce semifinished, wrought nickel products; and

(f) thereafter heating said strip or sheet at a tempera- 8 ture within the range of from about 1200 F. to about 1800 F. to reduce the hardness thereof to below 35 Rockwell T hardness.

4. As a new article of manufacture, wrought nickel strip produced in accordance with the method of claim 3, containing at least 99.8% nickel and containing less than 0.003% of sulphur, less than 0.005 of carbon and having a hardness below about Rockwell 30T hardness scale.

References Cited by the Examiner UNITED STATES PATENTS 2,159,231 5/1939 Schlecht et a1. 29420.5 2,159,604 5/1939 Schlecht et al. 224 2,826,805 3/1958 Probst et a1. 148-126 2,889,224 6/1959 Evans et al. 75214 2,900,254 10/1959 Raiklen 75214 OTHER REFERENCES Henderson, Metallurgical Dictionary, Reinhold Publishing Corp., New York 36, New York, 1953, p. 57.

DAVID L. RECK, Primary Examiner.

REUBEN EPSTEIN, Examiner.

R. L. GOLDBERG, H. F. SAITO, Assistant Examiners. 

1. THE PROCESS FOR PRODUCING NICKEL SHEET AND STRIP HAVING A DENSITY OF ABOUT 100% OF THE THEORETICAL DENSITY AND A HARDNESS BELOW ABOUT 35 ROCKWELL 30T HARDNESS SCALE WHICH COMPRISES THE STEPS OF: (A) ROLL COMPACTING NICKEL PARTICLES OF RANDOM SIZES SMALLER THAN ABOUT 300 MICRONS TO PRODUCE A GREEN STRIP OR SHEET HAVING A DENSITY LESS THAN 10% OF THE THEORETICAL DENSITY; (B) SINTERING SAID STRIP IN A FLOWING WET HYDROGEN ATMOSPHERE AT A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 1500*F. TO ABOUT 2100*F. TO LOWER THE SULPHUR CONTENT OF SAID STRIP TO BELOW ABOUT 0.003% BY WEIGHT AND THE CARBON CONTENT OF SAID STRIP TO BELOW ABOUT 0.005% BY WEIGHT; (C) HOT ROLLING SAID SINTERED STRIP OR SHEET TO INCREASE THE DENSITY THEREOF TO ABOUT 100% OF THE THEORETICAL DENSITY; (D) COOLING SAID STRIP OF SHEET UNDER A PROTECTIVE ATMOSPHERE PHERE TO ABOUT NORMAL ROOM TEMPERATURE; (E) COLD WORKING SAID STRIP OR SHEET; AND (F) THEREAFTER HEATING SAID STRIP OF SHEET AT A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 1200*F. TO ABOUT 18000*F. FOR A PERIOD OF TIME SUFFICIENT TO REDUCE THE HARDNESS THEREOF TO A SELECTED VALUE BELOW 35 ROCKWELL 30T HARDNESS. 